bugfix: Normalizing literals in SELECT is broken

[systay]

Description

Vitess does auto-parameterization of queries. This means replacing literal values with arguments, so that a plan cache can be re-used between queries, if the only difference is the literal value.

Sometimes, this rewrite messes with queries, such as in the issue reported below.

In this PR, I wanted to stop the Normalizer from rewriting expressions if they are inside unalised select expressions.

To do that, first I needed to clean up our AST a little bit.

Back in the days, we used FuncExpr for all expressions, including count(*). Since then, we have replaced most functions with custom AST types (such as CountStar), and this means that we no longer need to parse * as an argument to a function call.

Using pure expressions simplifies a lot of code and makes it possible to know that when we are looking at an *sqlparser.AliasedExpr, we know we are in the SELECT clause and nowhere else.

Related Issue(s)

Fixes #15020

Checklist

• [x] "Backport to:" labels have been added if this change should be back-ported to release branches
• [x] If this change is to be back-ported to previous releases, a justification is included in the PR description
• [x] Tests were added or are not required
• [x] Did the new or modified tests pass consistently locally and on CI?
• [x] Documentation was added or is not required

Deployment Notes

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[systay]

@GuptaManan100 I don't know how to handle this. I think our old way of parsing was hiding an invalid parse output

select 'a' 'b'

was being parsed as

select 'a' as b

but if we look at how mysql treats it, we see this:

mysql> explain select 'a' 'b';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra          |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
|  1 | SIMPLE      | NULL  | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL |     NULL | No tables used |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
1 row in set, 1 warning (0.00 sec)

mysql> show warnings;
+-------+------+-----------------------------------+
| Level | Code | Message                           |
+-------+------+-----------------------------------+
| Note  | 1003 | /* select#1 */ select 'ab' AS `a` |
+-------+------+-----------------------------------+
1 row in set (0.00 sec)

[GrahamCampbell]

Needs a backport to 19.0 label?

[systay]

Needs a backport to 19.0 label?

Not sure we want to backport this. @deepthi @harshit-gangal wdyt?

[codecov[bot]]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (4e31f60) 67.29% compared to head (08cabf1) 67.27%.

Files	Patch %	Lines
go/vt/vtgate/planbuilder/operators/aggregator.go	0.00%	5 Missing :warning:
go/vt/vttablet/tabletmanager/vdiff/table_differ.go	0.00%	4 Missing :warning:
go/vt/vtgate/simplifier/expression_simplifier.go	0.00%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15043      +/-   ##
==========================================
- Coverage   67.29%   67.27%   -0.02%     
==========================================
  Files        1560     1560              
  Lines      192123   192109      -14     
==========================================
- Hits       129283   129249      -34     
- Misses      62840    62860      +20     

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[frouioui]

Not sure we want to backport this. @deepthi @harshit-gangal wdyt?

I feel like we should given this is a bug fix, but i have no strong opinions.

EDIT: I think we should also backport to release-18.0, the linked issue (https://github.com/vitessio/vitess/issues/15020) was using v18.0.1.

[harshit-gangal]

Needs a backport to 19.0 label?

Not sure we want to backport this. @deepthi @harshit-gangal wdyt?

some part of it is regression from v17 and some are new support. So, we should backport it till v18